1. Field of the Invention
The present invention is directed to an apparatus for transporting single sheets through a device for exposing or printing single sheets. The apparatus includes a print drum and is constructed in such a way that a single sheet is pressed against the print drum. A processing station is associated with the print drum which is formed to include a processing gap. The processing gap includes a mechanism for limiting the processing gap so that the distance between such mechanism and the print drum surface can be increased or decreased.
2. Description of the Related Art
U.S. Pat. No. 5,968,714 discloses an apparatus for recording information on photographic material that can be thermally developed. The apparatus includes a heated drum which deflects each single sheets by approximately 180xc2x0. In a first segment of the deflection, the single sheet is urged against the drum by four pressure rollers. An exposure gap is located between these pressure rollers, with a laser recording on the single sheet through this gap. After the exposure step, the pressing operation is taken over by a continuous belt which wraps around the drum in this stage of the deflection. The continuous belt is provided to maintain the single sheets in close contact with the drum so as to ensure an optimal heat transfer between the drum and the single sheet.
It is the object of the invention to form means for guiding single sheets on a drum past a processing gap so that both the leading edge and the trailing edge of the single sheets can be transported past the processing gap unhindered and without hitting anything, thereby ensuring a continuous, jerk-free motion of the single sheets across the processing gap.
The object is solved by an apparatus having means for limiting the processing gap which are moveable in such a way that the distance between the drum surface and the means for limiting the processing gap is increased or decreased. The internal stress of the individual sheets lifts both the leading edge and the trailing edge from the drum during transport across the exposure gap as soon as the respective edge has passed the means for limiting the processing gap. This sudden lifting causes synchronizing errors in the processing station. For example, when photographic paper is exposed in the processing gap with a laser beam, this sudden lifting causes noticeable image defects. The movability of the means for limiting the processing gap can prevent a sudden lifting of the single sheets from the drum and can therefore significantly improve the image quality in a laser exposure station.
Advantageously, the diameter of the drum can be selected to be no less than 150 mm. For smaller diameters, for example, when using photographic paper, the force produced by the internal stress of the photographic paper can be so large as to urge the leading edge and the trailing edge in the exposure gap away from the drum surface. Likewise, even if the exposure gap were very narrow, the lack of guiding in the exposure gap can change the spacing between the drum surface and the leading and trailing paper edge, so that the image quality may no longer be acceptable.
Advantageously, a plurality of parallel flat belts maintains contact between the single sheets and the drum surface both in front of and behind the processing gap. The means for limiting the processing gap have the form of a comb and are located in front of and behind the processing gap, with the individual teeth of the comb projecting in the space between the belts.
An optimal result can be attained if the means for limiting the processing gap in front of and behind the processing gap are moveable.
In the open, raised position, both combs advantageously form a tapered gap with the drum surface. The wider portion of the gap has a dimension so that the distance between the comb and the drum surface is always greater than the distance between the drum surface and the leading or trailing edge of the single sheet lifted from the drum surface by the internal stress.
To keep the region of the processing gap where the leading edge and the trailing edge of the single sheets are not guided, as small as possible, the comb positioned behind the processing gap can preferably be moved into the raised position, in front of the leading edge of a single sheet leaves the processing gap, but not in front of the trailing edge of a previously processed single sheet has passed the comb.
As soon as the leading edge of single sheet is transported past the comb positioned behind the processing gap and thereby inserted between the drum surface and the flat belts, this comb can be moved into the lower position.
In front of the trailing edge of the single sheet reaches the comb positioned in front of the processing gap, this comb can be moved into the raised, open position. In this way, the trailing edge of the single sheet is not suddenly lifted from the drum surface, but proceeds slowly and continuously to contact the comb in the raised position.
Advantageously, only one comb is in the open position and lifted off the drum surface at any given time. With this arrangement, the region where the single sheet is not guided, can be kept very small.